1. Field of the Invention
The present invention relates to a power circuit which outputs and applies an AC voltage and a DC voltage to respective terminals of a capacitor, so as to obtain an AC output voltage shifted in accordance with the DC voltage component.
2. Description of the Related Art
In recent years, LCDs (liquid crystal displays) characterized by a flat shape and low power consumption are employed in a wide variety of forms ranging from a small-sized LCD mounted on a mobile phone to a large-sized LCD such as a wall-mounting television panel. In an LCD, voltage is applied to liquid crystal to change the state of alignment of the liquid crystal, thereby adjusting light transmittance and controlling display.
Such voltage application to liquid crystal may be performed according to an active matrix scheme using a thin film transistor (hereinafter referred to as “TFT”) as disclosed in Japanese Patent Laid-Open Publication No. 2000-81606. The active matrix scheme is explained referring to FIG. 4. In an active matrix panel, gate lines 300 extending along the row direction and drain lines 400 extending along the column direction are arranged intersecting one another, defining pixel areas. Each pixel includes a TFT 500 which serves as a switching element. A pixel electrode provided separately in each pixel is connected to the source of the TFT 500. A counter electrode 900 is disposed commonly opposing the pixel electrodes formed in the individual pixels over the entire panel. Further, liquid crystal is sealed between the pixel electrodes and the counter electrode 900. Accordingly, a liquid crystal element 600 for each pixel is composed of a part of the common liquid crystal positioned between the pixel electrode of an individual pixel and the counter electrode 900. Furthermore, an auxiliary capacitor 700 is provided at a connection point between the TFT 500 and the pixel electrode by coupling with an auxiliary capacitance line 800.
According to the active matrix scheme, a gate voltage for turning on the TFTs 500 is sequentially applied to each gate line 300. When the gate voltage is applied to a gate line, all of the TFTs 500 in the corresponding row are turned on to allow electrical conduction between the drain and the source. When the TFTs 500 of one row are turned on, video signals input into the drain lines 400 corresponding to those pixels are passed through the TFTs 500 and retained in the auxiliary capacitors 700, allowing the video signals to be applied to the pixel electrodes. In this manner, each pixel electrode is supplied with a voltage in accordance with a video signal corresponding to that pixel. When such voltages are applied to the liquid crystal 600 between the pixel electrodes and the counter electrode 900 in one row, an image in accordance with the video signals can be displayed for one horizontal scan line. By sequentially repeating this process for individual horizontal scan lines, a screen display can be achieved.
In an LCD, one screen image is displayed by allowing each auxiliary capacitor 700 to retain a voltage and maintain the potential of a corresponding pixel electrode, so as to continue applying the voltage to the liquid crystal 600 during one field.
Furthermore, an alternating current drive method has been proposed in recent years. According to this method, polarity of a video signal and polarity of the voltage of the counter electrode 900 are reversed every scan line.
FIG. 5 shows an example of a conventional power circuit for the alternating current drive method. This power circuit applies an AC voltage and a DC voltage to respective terminals of a capacitor, and outputs an AC voltage shifted in accordance with the DC voltage component for supplying to the counter electrode 900. The power circuit 110 of FIG. 5 comprises a voltage adjuster 11 for outputting an AC voltage, and a voltage adjuster 21 for outputting a DC voltage. The voltage adjuster 11 employs a switch element to perform switching between DC voltage Vw and the ground level (0V), so as to produce an AC voltage (pulse voltage). Further, a capacitor 200 is used to eliminate DC components, and the obtained AC voltage is supplied to output Out. Meanwhile, the DC voltage component V2 is output from the voltage adjuster 21 and supplied via a resistor 40 to the output Out. Using this arrangement, the AC voltage supplied by the voltage adjuster 11 can be shifted by the DC voltage supplied by the voltage adjuster 12, and the shifted AC voltage can be obtained at the output Out. The resistor 40 is provided to prevent the AC voltage output at Out from influencing the voltage adjuster 21.
When employing the above-described conventional power circuit to apply an AC voltage to the counter electrode of an LCD, it is necessary to adjust the frequency of the AC voltage to match the low-frequency switching timing of the horizontal scan lines of the LCD, which is typically about several ten Hz. For this reason, the capacitor connected to the power circuit must have relatively large capacitance in the order of several ten μF. Further, it is necessary to use a resistor having relatively large resistance in the order of several hundred kΩ as the resistor connected to the voltage adjuster 21. Accordingly, when turning off power of the conventional power circuit, a relatively long time is required before charges in the capacitor are completely discharged, therefore disadvantageously requiring a considerable time before the voltage applied to the counter electrode is converged to 0V.
Assuming that the internal resistance within the voltage adjuster 11 for outputting AC voltage is 0Ω, the capacitor value is 47 μF, and the resistor value is 100 kΩ, the conventional power circuit would require approximately two seconds before the capacitor is completely discharged by the voltage adjuster 21 to achieve 0V.